The default size can be overridden by CUPS in a number of ways, usually
with the help of lpoptions. This new size is also "correct".

It is correct because that is what is in the paper tray. There is no default
paper size setting for this printer through the CUPS web interface. I know
that is unusual but it is the way that printer's CUPS driver seems to work.

Scroll down the page to reveal the Page Size option.

You're right. It is set to letter.

Do you have an lpoptions file in /etc/cups or $HOME/.cups?

There is no lpoptions file in /etc/cups and the one in $HOME/.cups is empty.

It is okular without KDE here and it honours lpoptions. Try

lpoptions -p Samsung_C410_Series -o PageSize=Letter

Does that require a restart because it doesn't seem to fix the problem.
After I entered the command, $HOME/.cups/lpoptions contains "Dest
Samsung_C410_Series PageSize=Letter" but Okular still wants to use A4.